To avoid short-channel effects, P+ polysilicon is commonly used as the gate material for surface-channel P-type metal-oxide-semiconductor field-effect transistors (MOSFETs). However, the boron used to dope the polysilicon can penetrate through to the gate oxide and to the underlying silicon. In particular, fluorine atoms, associated with the BF.sub.2 ion implantation, enhance boron penetration through the thin gate oxide into the Si substrate. This results in a large threshold voltage shift, high charge trapping rate, degradation of P-channel inverse subthreshold and poor reliability of the devices.
Various techniques have been proposed to suppress the boron penetration. For example, use of an amorphous silicon gate has been suggested since it prevents the channeling effect and its larger grain size suppresses boron diffusion. A stacked gate has also been suggested since it has the ability to getter F atoms at its layer boundaries, thereby retarding boron diffusion.
The approach taken by the present invention has been to utilize argon implantation into the P+ poly-Si gate to suppress the boron diffusion. It is known that Ar-implantation in polysilicon or silicon generates bubble-like defects. These have the property of gettering the fluorine and boron atoms at the damaged regions due to Ar implantated into polysilicon or .alpha.-silicon thereby suppressing the boron penetration. The integrity of gate oxide is thus preserved and excellent electrical characteristics for the interface state density (D.sub.it), dielectric breakdown (E.sub.bd), and charge-to-break down (Q.sub.bd) of the gate oxide are obtained.
A general description of the argon implantation method can be found in Anjum et al. (U.S. Pat. No. 5,393,676 February 1995) which describes how a diffusion barrier comprised of implanted argon ions is placed deep within the gate material (which is limited to polysilicon). An argon ion dose between 10.sup.13 and 5.times.10.sup.14 per cm..sup.2 is specified. Anjum et al. locate their diffusion barrier at a depth of about 4,000 Angstroms, corresponding to an ion implantation energy of about 250 keV. Other references of interest include Aronowitz et al. (U.S. Pat. No. 5,468,974 November 1995) who teach that implanted species can be used to control diffusion of of donors or acceptors. Ngaoaram (U.S. Pat. No. 5,605,848 February 1997) uses nitrogen ions to prevent penetration of impurities into an oxide layer. Tsunoda (U.S. Pat. No. 5,518,943 May 1996) shows a gate process where argon ions are implanted into the substrate while Chen et al. (U.S. Pat. No. 5,576,228 November 1996) shows a method of forming a gate electrode.